1. Field of the Invention
The present invention relates to image forming apparatuses that form images on sheets from printers, facsimile machines, etc.
2. Description of the Related Art
In copying machines, printers, facsimiles, etc., very often the electro-photographic method is used in which images are formed on sheets using toners. Further, even color image forming apparatuses that form color images using toners of multiple colors are large in number, and recently, considering high productivity, tandem type color image forming apparatuses have appeared in which the photoreceptor, writing section, and developing section, etc., are provided for each color, and toner images of different colors are superimposed on one another in an intermediate image transfer member.
However, there is a demand from the users for outputting on sheets high quality images without any changes in time, and in said image forming apparatuses, compensation operations are made at regular intervals of time in order to stabilize the image quality. The compensation operations executed at regular intervals of time are, for example, the position shift compensation operation that corrects the writing timing in the writing sections so that the toner images of each of the colors of yellow (Y), magenta (M), cyan (C), and black (K) are superimposed on each other in the intermediate image transfer member without any shifts, or the operations of correcting the density or gray scale of the images, etc.
In order to execute compensation operations such as the position shift compensation operation or the image density compensation operation, etc., it is necessary to form image patterns for compensation on the photoreceptor or on the intermediate image transfer member, and to read out that image pattern using a sensor. However, stopping the print job that is currently being executed by the image forming apparatus in order to form the image pattern is not desirable from the point of view of productivity. Therefore, various types of technologies have been proposed for executing the compensation operations without stopping the print jobs.
The technology disclosed in Japanese Unexamined Patent Application Publication No. H10-213940 is a technology according to which, image patterns for color shift compensation are formed between the sheets in which images are formed on the image transfer belt (the non-image area), these image patterns are detected by sensors, and the phase of the polygon in the writing section is controlled. According to this technology, since there is no stopping of the print jobs executed in the image forming apparatus, it is possible to carry out color shift compensation operations while preventing a reduction in the productivity.
The compensation operations for stabilizing the image quality as described above are of many types, such as, position shift compensation operation, image density compensation operation, etc. The timing of executing these compensation operations is determined based on the number of times of image forming operations in the image forming apparatus, and it is possible that the timing of execution of different types of compensations coincide with each other.
In that case, it is possible to think of forming the image patterns for each of the different compensations in the non-image area as in the technology disclosed in Japanese Unexamined Patent Application Publication No. H10-213940, and also, for the image patterns for each of the different compensations in different non-image areas. However, if this is done, until the image pattern formed in the rear non-image area is detected by the sensor, it is not possible to execute the compensation operations based on that image pattern, and even though it has become possible to prevent a reduction in productivity in the image forming apparatus, the compensation operations for images gets delayed and it is possible that the image quality decreases.
In view of this, the purpose of the present invention is to provide an image forming apparatus in which the correction operations for stabilizing the image are executed without delay and also to prevent a reduction in productivity.